When an analog signal is transmitted to an LCD (Liquid Crystal Display Device), a CRT (Cathode Ray Tube), or the like, distortions in the wave form and the like generate blurring and ghosting on the display screen.
DVI Specification
In order to solve the above-described problem, in the DVI (Digital Visual Interface) Standard, video signals are sent digitally to LCDs, CRTs, and the like. This enables a screen to be displayed with high picture quality and without distortion.
Conventional signal transmission systems to which the DVI specification is applied are composed of a transmission device and a reception device which are connected via a transmission path. The transmission device has three TDMS (Transition Minimized Differential Signaling) encoders/serializers, and the reception device has three TDMS recovery/decoders. Component signals of each of RED, GREEN, and BLUE are input into the respective corresponding TDMS encoder/serializers, encoded therein, and the results are serialized and output to the transmission path. Next, each of the TDMS recovery/decoders in the reception device decodes the respective received signal, and recovers and reconstructs the respective component signal.
A DE (data enable) signal shows a period in which component signals RED, GREEN, BLUE, etc. exist, and is HIGH active. A period in which the DE signal is LOW is, for example, a horizontal synchronization signal period or a vertical synchronization signal period. CLT signals CTL0, CLT1, CTL2, and CLT3 are provided as control signals, but in the current DVI specification these signals are unused. Specifically, ordinarily the level of these signals is zero.
Each of the TMDS encoder/serializers in the transmission device converts an input 8-bit signal into 10 bits, and serializes and sends the converted 10-bit video signal in serial to the transmission path. The purpose of converting from 8 bits to 10 bits is that a 10-bit signal is suitable for high-speed transmission of data with fewer data change points. In addition, the TDMS encoder/serializers convert the 2-bit control signal to 10 bits and transmit the result to the transmission path. The data enable signal is also encoded, serialized and transmitted to the transmission path. The TDMS recovery/decoders in the reception device decode and expand the 10-bit serial data received from the transmission path into 8-bit color signals, a 2-bit data enable signal, and a 2-bit control signal.
HDCP Specification
An HDCP (High-bandwidth Digital Content Protection System) specification has been proposed as a digital content protection system that is compatible with the DVI specification.
The HDCP specification uses a signal transmission system that is compatible with the DVI specification to transmit video content requiring right protection. Essentially the HDCP specification consists of authentication between the transmission device and the reception device, sharing of a key, and encryption of video content over a communication path.
A transmission device in a signal transmission system that complies with the HDCP specification has an authentication unit that performs authentication and key exchange with the reception unit, an encryption unit that encrypts video information using a shared key, and a TMDS encoding unit. A reception unit has an authentication unit that performs authentication and key exchange with the transmission unit, a TDMS decoding unit, and a decryption unit that decrypts a received signal using the shared key.
According to this construction, after verification and key exchange via an I2C between the transmission device and the reception device, the transmission device encrypts video RGB data, and transmits the encrypted data via a TDMS encoder that complies with the DVI specification. After receiving the encrypted data via a TMDS decryption unit that complies with the DVI specification, the reception unit decrypts the encrypted video RGB data using the same key as that used by the transmission device, to obtain the original video RGB signal. In the HDCP specification the encryption used here is called “HDCP Cipher”. The core part of HDCP Cipher is common to authentication, key exchange, and video data encryption.
As explained above, a data transmission system that is compatible with DVI specifications and HDCP specifications enables transmission of high quality images while ensuring protection of right protected works over the transmission path.
In recent years it has become common to reproduce digital video to which digital audio is attached in personal computers, digital broadcast reception devices, DVD (Digital Versatile Disk) reproduction devices, and the like. Accordingly, there is now a demand for high-quality transmission of audio, similar to that described above for video.